The present invention relates generally to an imaging system utilizing imaging sheets containing rupturable photosensitive microcapsules and, more particularly, to a tension arm developer apparatus for developing images in contact with the imaging sheet by pressure development using a point contact element in contact with the outer surface of the imaging sheet.
U.S. Pat. Nos. 4,399,209; 4,416,966; 4,440,846; 4,842,981; 4,865,943; and 4,766,050, all commonly assigned to The Mead Corporation, describe an imaging system wherein a photosensitive layer comprising microcapsules containing a photosensitive composition in the internal phase is image-wise exposed to actinic radiation and subjected to a uniform rupturing force whereupon the microcapsules rupture and image-wise release the internal phase which reacts with a developer material to produce an image therein. The imaging system is particularly advantageous because it is a totally dry system and does not rely upon the application of wet development processing solutions to produce the image. An image forming chromogenic material, such as a substantially colorless color former, is typically associated with the microcapsules. When the microcapsules rupture, the color former image-wise reacts with a developer material and produces a color image. In the embodiments described in the referenced patents, the microcapsules are typically ruptured by passing image-wise exposed imaging sheets through the nip between a pair of calendar rollers.
While heavy pressure is not required to rupture the microcapsules, high pressure and large calender rollers are normally used to develop the imaging sheets. Even carefully machined metal calender rollers have uneven surfaces. If one roller is simply rested upon another, the surfaces of the rollers are not in contact over the entire length of the rollers. By applying pressure to the rollers, the uneven surfaces or surface irregularities are "smoothed out" to provide a uniform contact line between the rollers. The high pressure and large size of the rollers are necessary to achieve a uniform distribution of the rupturing forces across the surface of the imaging sheets. If the rupturing force is not uniformly distributed, the imaging sheets develop unevenly and the tonal characteristics of the resulting images are not good.
As the width of the imaging sheets and the corresponding length of the calender rollers increases, the diameter of the rollers must also be increased to maintain sufficient stiffness such that pressure applied to the ends of the rollers is distributed evenly along the entire expanse of the rollers. As a general rule, the diameter of the rollers must be doubled as the length of the rollers is doubled in order to maintain sufficient stiffness. Thus, as the size of the imaging sheets increases, the cost and size of effective pressure-type development apparatus becomes prohibitive. In particular, while larger pressure rollers may be accommodated in developing apparatuses designed to develop relatively small copies, e.g., 8-12 inches in width, in certain applications, such as color proofing, large copy widths sometimes in excess of 36 inches are used. These copies are so large as to make it impractical to develop them using pressure rollers.
An additional drawback of processing imaging sheets by means of high pressure calender rollers is that some of the midtone quality of the developed images is lost. This is apparently due to a loss of a differential microcapsule rupture and is referred to as "midtone mottle". Thus, a number of difficulties have been encountered in designing calender rollers for pressure development of the aforementioned imaging sheets.
Alternate techniques for processing imaging sheets without bulky, high pressure calender rollers are known in the prior art. In commonly assigned U.S. Pat. No. 4,448,516, the imaging sheets are passed over a developer roll having a fibrous outer surface composed of interwoven natural or synthetic fibers or flexible bristles. The developer roll is rotated in contact with the imaging sheets to rupture the microcapsules on the sheets.
A second arrangement for generating shear forces for processing imaging sheets is disclosed in commonly assigned U.S. Pat. No. 4,578,340, and entitled "Free Particle Abrasion Development of Imaging Sheets Employing Photosensitive Microcapsules," wherein imaging sheets are contacted with a bed of free particles such that the free particles pass over the microcapsules on the surface of the imaging sheets to rupture the microcapsules. A third arrangement is disclosed in U.S. Pat. No. 4,592,986, and entitled "Magnetic Brush Abrasion Development of Imaging Sheets Employing Photosensitive Microcapsules," wherein imaging sheets are processed by contacting the imaging sheets with a traveling pile formed by magnetically attractable free particles on a magnetic brush.
Commonly assigned U.S. Pat. No. 4,648,699 describes the development of an imaging sheet containing microcapsules by moving a point contact which is resiliently biased into engagement with the imaging sheet across the entire surface of the sheet. Typically, the imaging sheet is secured to a cylinder and a point contact is positioned in resilient pressure contact with the imaging sheet. As the cylinder is rotated, the point contact is simultaneously moved along the cylinder in synchronism with the rotation of the cylinder to rupture the microcapsules and develop the image in the imaging sheet or the imaging sheet may be mounted on a planer platform and the point contact is moved across the surface of the sheet using an X-Y transport device. Also disclosed is an apparatus for developing a continuous web in which the point contact traverses the surface of the web in a circular path.
While these alternate processing arrangements each offer attractive features and may be preferred for selected applications, newer methods and devices which offer simpler and less expensive techniques for developing imaging sheets are desirable.